Here is the problem with any car:
The more money you spend, the less the performance increases. Take a 911 for instance, it´s quite a bit more than the M3, but the 911S is pretty much as quick.
The 335 is almost 1/3 cheaper, and its barely slower. Any car manufacturer displays this problem, and the higher the league, the more expensive this little extra gets.
Now, looking at the value of the 335i, I can imagine a lot of people feel like it is a better deal for the money, and it probably is. This little extra in performance for the M3, the sweeter look, and of all M3 characteristics etc. simply have their price.
On the other hand, the M3 is still relatively cheap in comparison to other models (911), and if its worth it for you: Go for it!
It is for me, and I cannot wait to drive it.